1. Field of the Invention
This invention relates to a bearing device for a high speed rotary shaft which supports a rotary shaft rotating at a ultra-high speed.
2. Description of the Prior Art
A turbo-charger for effectively utilizing exhaust energy by disposing a motor-generator to a shaft of a turbine driven by exhaust energy of an engine is disclosed, for example, in Japanese Patent Laid-Open No. 51723/1987. Such a turbo-charger will be explained with reference to FIG. 7 of the accompanying drawings. The turbo-charger 61 comprises a turbine 63 driven by exhaust gas energy of an internal combustion engine and a compressor 62 which is driven by the turbine 63 and supercharges air into a cylinder. It also includes a motor-generator 64 driven by the turbine 63 and moreover, the turbine 63, the motor-generator 64 and the compressor 62 are disposed sequentially on a shaft 66. The motor-generator 64 is operated either as a motor or as a generator in accordance with the operating condition of the internal combustion engine. The number of revolution of the shaft 66 of the turbo-charger 61 ranges from a low speed revolution range to a ultra-high speed revolution range of about 150,000 to 200,000 rpm, from time to time. In this turbo-charger 61, a blade 69 of the turbine 63 is disposed at one of the ends of the shaft 66 and an impeller 68 of the compressor 62 is fitted to the other end, and the motor-generator 64 having a rotor 71 and a stator winding 72 is disposed between the blade 69 and the impeller 68. A pair of bearings 65, 65 are disposed on the shaft between the blade 60 of the turbine 63 and the motor-generator 64 in order to support rotatably the shaft 66 on a housing 70. A lubricating oil or lubricant is supplied to the bearing portion of these bearings 65, 65 through a lubricant passage 67 formed on the housing 70 to lubricate them.
In the structure described above wherein the shaft 66 is supported by the pair of bearings 65, 65, the support portion of the bearings 65 is one-sided and the great masses such as the rotor 71 of the motor-generator 64 and the impeller 68 of the compressor 62 are disposed at the other end, the rotation-supporting state of the shaft 66 does not cause any trouble as represented by solid line a in FIG. 5 when the shaft 66 rotates at a low speed of revolution such as below 120,000 rpm, but the following problems occur when it rotates at a high speed revolution. When the shaft 66 rotates at a high speed revolution such as 120,000 rpm or more, for example, the centrifugal force becomes great due to the high speed rotary members such as the rotor 71 and the impeller 68 so that unbalance occurs in the shaft 66 and deflection of the shaft 66 becomes great as represented by dash line b in FIG. 5. Therefore, balance correction of the shaft 66 becomes necessary in order to prevent deflection of the shaft 66. The natural frequency of the shaft drops and resonance number of revolution drops, too, so that amplitude of deflection becomes great.
For the reasons described above, the acceptable range as the maximum value of the normal number of revolution is 120,000 rpm, for example, in the case of two-point structure where the shaft 66 is supported by two bearings 65 but when the number of revolution of the shaft 66 increases and enters a high speed revolution range, the deflection of the shaft 66 becomes so great that the revolution of the shaft 66 cannot be raised to a rated number of revolution such as 150,000 rpm, for example.
In order to let the shaft withstand the high speed revolution and to prevent the occurrence of deflection of the shaft, therefore, counter-measures have been attempted by by increasing the diameter of the shaft or selecting a suitable material to improve rigidity, reducing the weight of adding masses such as the stator and the impeller or making ultra-precision correction of the unbalance quantity. If such counter-measures are employed, however, friction increases between the components rotating relative to one another and response such as the rotation and stop of the shaft and its rotation control gets deteriorated. Moreover, efficiency of the turbo-charger and the motor-generator drops, producibility drops also and the cost of production increases.